add warning to dnnlowp fc if quantization kind is not min_max
Summary:
Print warning when using DNNLOWP dynamic int8 quant for FC and activation_quantization_kind != min_max.
Warning will display in console but not in Bento. Would have to use CAFFE_ENFORCE to alert in Bento.
Test Plan: buck run unit test forcing DNNLOWP FC with activation_quantization_kind = "l2" and saw warning printed in console.
Reviewed By: csummersea
Differential Revision: D17770921
fbshipit-source-id: b6532e4c9a86d74e3db4cb432735505d378a366e
